Life on Earth is carbon-based, as all macromolecules that make up living organisms contain carbon atoms. All organic compounds have a carbon backbone. Each carbon atom is tetravalent and can bond with four other atoms, making it an extraordinarily flexible component of biological molecules. Because carbon’s valence electrons are stable, it rarely becomes an ion. As the carbon chain increases in length, structural modifications such as ring structures, double bonds, and branching side...

Early Atmosphere And OceansEarth’s atmosphere and oceans have changed significantly since the planet formed about 4.6 billion years ago. In its early history, Earth had a thick atmosphere composed primarily of carbon dioxide, methane, and water vapor, but it lacked oxygen. The first oceans formed as volcanic eruptions released gases and water vapor condensed. Over time, photosynthetic organisms began producing oxygen, gradually transforming Earth’s atmosphere into what we have today.

Atoms and MoleculesEverything around us is made of tiny particles called atoms. Atoms are the smallest units of matter and are the building blocks of all substances. When atoms bond together, they form molecules, which make up solids, liquids, and gases. Understanding atoms and molecules helps scientists explain how different substances interact and change in both physical and chemical processes.Science and Engineering Practices (SEP): Developing and Using ModelsScientists use models to...

All living things are formed mostly of carbon compounds called organic compounds. The category of organic compounds includes both natural and synthetic compounds that contain carbon. Although a single, precise definition has yet to be identified by the chemistry community, most agree that a defining trait of organic molecules is the presence of carbon as the principal element, bonded to hydrogen and other carbon atoms. However, some carbon-containing compounds such as carbonates, cyanides, and...

Chemistry of CompoundsA chemical compound is a substance made up of two or more different elements that are chemically bonded together. When elements combine to form compounds, their atoms rearrange, resulting in new physical and chemical properties distinct from those of the individual elements.Science and Engineering Practices (SEP): Analyzing and Interpreting DataScientists create models of compounds to show how atoms bond and how molecular shapes affect their properties. Scientists can...

Molecular CompoundsMolecular compounds form when two or more nonmetal atoms share electrons through covalent bonds. Unlike ionic compounds, where electrons are transferred, molecular compounds remain electrically neutral because electrons are shared equally or unequally between atoms.Common Examples of Molecular Compounds:Water (H₂O): Essential for life; it forms hydrogen bonds that give it unique properties.Carbon dioxide (CO₂): A gas used by plants in photosynthesis.Methane (CH₄): A...